Method of filling hollow spaces with powdery insulating material



1961 J. c. DURAN EIAL 3,007,292

METHOD OF FILLING HOLLOW SPACES WITH PQWDERY INSULATING MATERIAL Filed Feb. 25, 1959 INVENTORS JOHANNES CHRISTIAAN DURAN HENRlCUS MARIA KQ-IJBOLDER AGE United States Patent i 3,007,292 METHOD OF FILLING HOLLOW SPACES WITH POWDERY INSULATING MATERIAI: Johannes Christiaan Duran and Henricus Maria Kn bolder, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Feb. 25, 1959, Ser. No. 795,532 Claims priority, application Netherlands Mar. 18, 1958 3 Claims. (Cl. 53-21) The invention relates to a method of filling hollow spaces with a powdery insulating material to a high filling factor and, in particular, to the making of an indirectly heated cathode having an insulating filling.

In many cases, for example with indirectly heated cathodes, it is desirable to fill the space between the heating element and the container of the emissive cathode material with powdery insulating material. It is known to introduce the powdery material into the space by producing a sediment of this material or by centrifugal force. It appears, however, that these known methods yield only a filling factor of less than 70%. By ramming and by a vibration method, the filling factor may be increased to only about 72%.

It has been found, however, that a much higher filling factor can be attained in an extremely simple manner, in accordance with the invention, by filling the space with a homogeneous suspension of the insulating powder in the charged state in a quantity of liquid of not more than 20% by weight and by subsequently evaporating of the liquid.

If the powder particles are sufficiently small, a filling factor of 80% may thus be reached without difficulty. The powder favourably contains a mixture of particles of which 90% is smaller than microns, and 10% is smaller than 2 microns, as is usually obtained by grinding and passing the powder through a 250 mesh screen. The liquid may consist of any liquid containing alcoholic OH groups, such as water, alcohol, monoethylglycolether, or the like, in which a suitable electrolyte is dissolved. As electrolytes can be used: acids with a monovalent anion, such as HCl, HNO further acidic reacting salts producing monovalent anions in the liquid concerned, salts with multivalent cations being preferred, such as the chlorides or nitrates of aluminum, zirconium, thorium or the like. The quantity of liquid, which is at the most 20% by weight of the suspension, does not suffice to form a liquid paste with powder in the uncharged state. In this case a brittle, unmanageable mass is obtained.

Colloid chemical charges are a phenomenon known per se, but as a rule, it is not desired. In this state the insulating particles are surrounded by an extremely thin, electrolytic double-layer, so that they repel one another and glide one over the other at a slight pressure, in spite of the almost dry state, as in the case of a liquid which does practically not wet objects. However, when the electrolyte is removed, the particles adhere extremely strongly to one another. Though the charged state is not dependent on the particle size, the effect is greatest with smaller particles.

The invention will be described more fully with ref- 3,007,292 Patented Nov. 7, 1961 ice erence to one example and to a drawing of which the figure shows a cathode manufactured in accordance with the invention.

Example Alumina is ground in a ball mill for 15 to 40 hours so that a powder is obtained consisting of 90% of particles of 10 microns and of 10% of particles of 2 microns. The mass, which contains iron owing to wear of the mill, is then purified with hydrochloric acid, washed thoroughly with water and then dried. The powder thus treated becomes susceptible to charging when a suitable liquid is added.

1000 gs. of this powder, with a volume of about 250 ccms., is mixed with 160 ccms. of distilled water, to which may be added 1 com. of 1 N AlCl It appears that a charged suspension is obtained, which behaves as a liquid and can be poured simply into a hollow space. After the water from the mixture has been evaporated, the filling factor is found to be to In order to obtain the same, thin-liquid properties of the said quantity of powder, an uncharged suspension would require approximately four times the quantity of water. A space filled therewith would have a filling factor of not more than 60 to 68%, even if centrifugal force is used or if the substance is caused to settle, after the water had been evaporated.

The figure shows a cathode, of which the emissive part 1 is supported by a metal cylinder 2, which encloses a heating body 3. The space inside the cylinder 2 is filled with alumina 4, which has a filling factor of more than 75%. Thus a satisfactory transfer of heat is ensured between the heating body 3, the cathode 1 and the cylinder 2. It appears that the small quantity of liquid can be evaporated very readily after the charged suspension has been poured in.

What is claimed is:

1. A method of making an object containing a space filled with alumina with an 80% or higher filling factor, consisting of a mass of alumina particles of which approximately are approximately 10 microns in size and the remainder are about 2 microns in size, mixing said mass of particles with less than 20% by weight of a liquid electrolyte to form a charged suspension, introducing into the said space said charged suspension and thereafter evaporating the liquid.

2. A method as set forth in claim 1 wherein the liquid comprises a substance containing alcoholic OH groups in which an electrolyte is dissolved.

3. A method as set forth in claim 1 wherein the electrolyte comprises a substance selected from the group consisting of acids and acidic-reacting salts.

References Cited in the file of this patent UNITED STATES PATENTS 1,754,003 Johnson-Vea et al. Apr. 8, 1930 2,079,231 Smith May 4, 1937 2,376,840 Wiegand May 22, 1945 2,542,657 Gall Feb. 20, 1951 2,624,500 Merrill et al. Jan. 6, 1953 

1. A METHOD OF MAKING AN OBJECT CONTAINING A SPACE FILLED WITH ALUMINIA WITH AN 80% OR HIGHER FILLING FACTOR, CONSISTING OF A MASS OF ALIMINIA PARTICLES OF WHICH APPROXIMATELY 90% ARE APPROXIMATELY 10 MICRONS IN SIZE AND THE REMAINDER ARE ABOUT 2 MICRONS IN SIZE, MIXING SAID MASS OF PARTICLES WITH LESS THAN 20% BY WEIGHT OF A LIQUID ELECTROLYTE TO FORM A CHARGED SUSPENSION, INTRODUCING INTO THE SAID SPACE SAID CHARGED SUSPENSION AND THEREAFTER EVAPORATING THE LIQUID. 